Engineering Craft › Developer Tooling

Linter

A tool that flags likely bugs and style issues.

Also known as: linting, lint, ESLint, Ruff, Pylint, flake8

A linter reads your code without running it and flags likely bugs, risky patterns and style problems.

// ESLint output
 3:7  error  'total' is assigned a value but never used   no-unused-vars
 9:5  error  Expected '===' and instead saw '=='           eqeqeq

npx eslint .        # JavaScript / TypeScript
ruff check .        # Python (others: flake8, pylint)

What they catch

• Unused variables and imports.
• Use of an undefined name (often a typo).
• Unreachable code.
• Risky patterns, like == in JavaScript, bare except: in Python, or comparing to None with ==.
• Team conventions: naming and import order.

A linter is a tireless reviewer for the boring things, leaving humans to look at design and logic.

Linter vs formatter

A formatter rewrites layout. A linter reports problems, and some can auto-fix the simple ones (--fix). Use both.

Using it well

• Run it in your editor so you see warnings as you type.
• Run it in CI so rules apply to everyone.
• Fix warnings; don’t silence them. If you disable a rule on a line, add a comment saying why.
• Start with a recommended preset rather than turning on every rule. Too many noisy rules get ignored.
• Treat a warning as a question. Sometimes the linter is wrong, but check first.

Linters are a light form of static analysis; heavier tools do deeper checks such as type analysis and security scanning. They won’t tell you whether your program does the right thing; that’s what tests are for.